In U.S. Pat. No. 4,334,628, by Buhrer et al, and U.S. Pat. No. 4,366,410, by Buhrer, both patents being incorporated herein by reference, there is disclosed a vacuum-tight assembly, such as a discharge tube for a metal vapor discharge lamp. The vacuum-tight assembly comprises a high-density polycrystalline ceramic body, such as alumina or yttria, having a cavity, at least one closure member, and a sealing material for hermetically sealing the cavity. The materials for the closure member and sealing material have thermal coefficients of expansion closely matched to the thermal coefficient of expansion of the ceramic body over a wide temperature range thereby facilitating fabrication of the vacuum-tight assembly by sintering. Certain metals and alloys are well suited for the closure member, e.g., molybdenum, or mixtures thereof. These refractory metals are especially difficult to weld. The closure member will hereinafter be referred to as an electric feedthrough assembly or simply as a feedthrough.
Various methods of fabricating electric feedthrough assemblies are known. The Buhrer et al patent teaches bonding of the electrode pin and the lead-in pin into holes in opposite sides of the body of the feedthrough by means of sintering or certain welding techniques which are not suitable for presently preferred refractory metals. The Buhrer patent teaches welding in an inert gas by an electric arc or laser welding.
In production, separate attachment of the tungsten electrode to the feedthrough is preferred over sintering-in of the electrode pin. Separate attachment allows a smaller part inventory for a particular feedthrough size. Separate attachment also permits easy adjustment of the desired backspacing for specific lamps.
In the existing art, separate attachment of the tungsten electrode to a niobium metal feedthrough is accomplished by tungsten-inert-gas (TIG) welding producing a brittle tungsten-niobium alloy. With the high temperatures experienced during TIG welding, extensive regions of the feedthrough are heated to the molten state and recrystallized during cooling. This also occurs in TIG welding of molybdenum metal feedthroughs. Extensive recrystallization produces larger grain size which often results in a brittle weld. Also, the recrystallized feedthrough body is prone to crack due to internal forces caused by temperature extremes experienced during lamp warm up, operation, and cool down. Where a titanium and molybdenum alloy is involved, oxidation is a persistent problem despite the inert gas environment; it is believed that oxygen may be released from the materials themselves which reacts with the alloy resulting in a weak mechanical connection with high electrical resistance. If the coil is attached to the electrode, an electron-emissive coating on the coil may be damaged by the welding process.
With laser welding carried out on molybdenum-titanium-nickel alloys, the joint to be welded is rotated under the beam of a pulsed YAG laser at 1.06 micrometers whereby considerable energy (and therefore heat) is absorbed by the joint. As a result, the same problems are experienced with laser welding as are experienced with TIG welding. Another disadvantage of laser welding is the high cost of the welding equipment.
While TIG and laser welding for niobium metal is being successfully employed in the lamp-making field today, it would be an advancement of the art if a novel welding technique were provided which avoids the difficulties and problems discussed above particularly if the new technique is economically feasible for manufacturing processes.